netdev - Re: Linux ECN Handling

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Date:   Wed, 3 Jan 2018 14:39:29 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Steve Ibanez <sibanez@...nford.edu>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>, Florian Westphal <fw@...len.de>,
        Mohammad Alizadeh <alizadeh@...il.mit.edu>,
        Lawrence Brakmo <brakmo@...com>
Subject: Re: Linux ECN Handling

On Tue, Jan 2, 2018 at 6:57 PM, Steve Ibanez <sibanez@...nford.edu> wrote:
> Hi Neal,
>
> Sorry, my last email was incorrect. It turns out the default tcp
> congestion control alg that was being used on my client machines was
> cubic instead of dctcp. That is why tp->processing_cwr field was never
> set in the tcp_rcv_established function. I've changed the default back
> to dctcp on all of my machines.
>
> I am now logging the value of tp->rcv_nxt at the top of the
> tcp_transmit_skb() function for all CWR segments. I see that during
> normal operation, the value of tp->rcv_nxt is equal to the SeqNo in
> the CWR segment  + length of the CWR segment.

OK, thanks. That makes sense.

This part I didn't understand:

> However, for the unACKed
> CWR segment, the value of tp->rcv_nxt is just equal to the SeqNo in
> the CWR segment (i.e. not incremented by the length). And I see that
> by the time the tcp_ack_snd_check() function is executed, tp->rcv_nxt
> has been incremented by the length of the unACKed CWR segment.

I would have thought that for the processing of the skb that has the
CWR, the sequence would be:

(1)  "...the tcp_ack_snd_check() function is executed, tp->rcv_nxt has
been incremented by the length of the unACKed CWR segment"

(2) then we send the ACK, and the instrumentation at the top of the
tcp_transmit_skb() function logs that rcv_nxt value (which "has been
incremented by the length of the unACKed CWR segment").

But you are saying "for the unACKed CWR segment, the value of
tp->rcv_nxt is just equal to the SeqNo in the CWR segment (i.e. not
incremented by the length)", which does not seem to match my
prediction in (2). Apparently I am mis-understanding the sequence.
Perhaps you can help clear it up for me? :-)

Is it possible that the case where you see "tp->rcv_nxt is just equal
to the SeqNo in the CWR segment" is a log line that was logged while
processing the skb that precedes the skb with the CWR?

> The tcp_transmit_skb() function sets the outgoing segment's ack_seq to
> be tp->rcv_next:
>
> th->ack_seq             = htonl(tp->rcv_nxt);
>
> So I think the rcv_nxt field is supposed to be incremented before
> reaching tcp_transmit_skb(). Can you see any reason as to why this
> field would not be incremented for CWR segments sometimes?

No, so far I haven't been able to think of a reason why rcv_nxt would
not be incremented for in-order CWR-marked segments...

cheers,
neal